RFID systems are widely used in the automotive industry and other industries such as farm animal identification, building access control, and meter reading. A primary application in the automotive industry is RFID used in the portion of the anti-theft system. The anti-theft system prevents a vehicle from being started without a key or a remote control device that has been electronically paired to a specific vehicle using RFID. RFID systems often include low power radio transponders. The transponder receives a radio signal from a base unit (also known as a “reader”). The reader may be built into a vehicle or other type of system. The transponder can be powered either by an energy source such as a battery or by energy harvested from the radio frequency (RF) field produced by the reader. Some transponders are primarily powered by a battery, with the added capability of being powered by the RF field when the battery cannot provide sufficient energy or when the battery is absent.
Upon receiving an RF signal from the reader with specific characteristics, the transponder responds to the reader by transmitting an RF signal with specific characteristics. The RF signals generated by the reader and transponder are typically modulated with data. The data transmitted by the reader and transponder is system dependent. Data transmitted by the transponder may include a transponder identification code, the reading of a water meter, or data from a vehicle key to be validated by the anti-theft system. A transponder in a vehicle key can be considered an “immobilizer” because, if the reader fails to receive the correct response from the transponder in the key or a key fob, the vehicle or equipment is disabled and may not start.
FIG. 1 is a block diagram of an example RFID system 100 operating at low radio frequency (LF). An example RF frequency employed in LF RFID systems is 134 kHz. Frequencies other than 134 kHz can be used. The “transponder” (block 101 of FIG. 1) operates in response to a query issued by a “reader” (block 103 of FIG. 1). Readers and transponders operating at low radio frequencies typically employ magnetically coupled antennas in close proximity. Initially in a communication session, the reader generates an LF field at approximately the natural resonant frequency of transponder antenna tank circuit 105 of FIG. 1. The LF field generated by the reader supplies energy to the transponder by inducing an oscillation in the transponder antenna tank circuit 105 of FIG. 1. When the transponder is being powered by the LF field, the oscillation induced by the reader in the transponder antenna tank circuit is typically rectified by the transponder. This rectified signal is typically used to charge a capacitor that powers the transponder. The reader also modulates the LF field with data for the transponder. Amplitude shift keying (ASK) modulation is typically used by the reader to transmit data. Data transmitted from the reader to the transponder is the “downlink” data. The transponder demodulates and processes the downlink data. The reader stops generating the LF field after completing the downlink transmission and after sufficient time has elapsed for the transponder to store sufficient energy to respond to the reader. After processing the downlink data, the transponder responds to the reader with data typically using frequency shift keying (FSK) modulation of the LF field. The LF field generated by the transponder and the reader are typically at approximately the same frequency. The same antenna tank circuit is typically used by the transponder for both receive and transmit. Data transmitted by the transponder to the reader is the “uplink” data. The group of radio signals 107 of FIG. 1 represent the energy in the LF field supplied by the reader, the downlink data, and the uplink data.
Transponders can be implemented with multiple antennas. For example, three orthogonally positioned antennas are often used in RFID transponders where the relative orientation of the reader and transponder is not fixed. Transponders implemented with multiple antennas often have separate receive and transmit circuits dedicated to each antenna in addition to common circuits. The common circuits typically include control functions, data processing, and power supply.
When the reader completes the downlink transmission and stops generating the LF field, transponders typically employ an oscillator to sustain oscillation in the antenna tank circuit (105 of FIG. 1) by adding energy to the antenna tank circuit. The energy added to the tank circuit by the oscillator periodically increases the peak-to-peak voltage range of the antenna tank circuit oscillation.
Transponders typically employ voltage limiting circuits coupled to the antenna tank circuit (105 of FIG. 1). This voltage limiting is employed to protect transponder circuit elements and insure the antenna voltage is within a range that can be processed by the transponder.